1. Field of the Invention
This invention relates to wireless communication systems and in particular to timing and delay periods in wireless communication systems.
2. Description of the Related Art
Wireless communication systems utilize a timeout period in which the base station expects to hear from the mobile station. If the base station does not hear from the mobile station within the timeout period, then the mobile station assumes the transmission has ceased. In addition, time division multiple access (TDMA) wireless systems assign transmission time slots to mobile stations and track the delay of each mobile station. In such systems, transmission timing of the mobile stations are adjusteds, forward or backwards, based on a reception of transmissions from the mobile station to maintain all the digital time slots synchronized and prevent adjacent users from having signals that overlap each other when they arrive at the base transceiver station (BTS).
For example, if user 2 in timeslot 2 is located directly under the cell site (where the base transceiver station is located) and user 1 is many miles away from the base station and is transmitting on timeslot 1, the time that it takes for the signal to arrive from user 1 at the base transceiver station is longer than the time it takes for user 2 transmissions to arrive given their different locations with respect to the cell site. The problem can arise that unless some timing adjustment is made for when user 1 should transmit, that the signal from user 1 in timeslot 1 can start to “overlap” in time with that being transmitted from user 2 in timeslot 2. The solution utilized by current systems is for user 1 in timeslot 1 to “advance” its timing, thereby causing user 1 transmissions to arrive earlier and ensuring that they do not interfere with timeslot 2.
For example, in Global System for Mobile Telecommunications (GSM) the delay is know as Timing Advance (TA), which has allowable values of 0, . . . , 63. One TA unit is equal to one GSM bit (3.7 μs). The electromagnetic waves travel at 1.1 Km/bit. That results in a round trip distance of 1.1×64=70 Km or 35 Km cell radius or a round trip delay of 64×3.7 μs=236 μs. Therefore in GSM the maximum cell size is limited to 35 kilometers, as this is the maximum timing advance that is allowed.
Referring to FIG. 1, a traditional architecture is illustrated that has distributed base transceiver stations 101 around the network 100 with multiple T1's 103 to each site. A base station controller (BSC) 105 controls multiple base transceiver stations 101. While the traditional architecture is functional for current needs, it lacks the ability to share radio resources efficiently. For example, in one major metropolitan network, the busy hour traffic carried is about 26 K Erl., while the network has 60 K voice paths (while being ˜99% digital). That suggests an approximately 26/60=43% efficiency in voice paths (capital).
Consequently, while the mobile switching center (MSC) switch may handle a certain load (simultaneous voice paths) during the busy hour, it is very common that it takes double the amount of effective voice paths in the deployed BTS's to handle the same net load on the switch. This is due to two reasons; 1) the individual cells have slightly different busy hours, and 2) there are no shared resources across BTS's for improving the trunking efficiency.
With new spectral efficiency improvements such as advanced multi-rate (AMR) in Global System for Mobile Telecommunications (GSM), and the newly approaching 3G technologies such as Enhanced Data Rates for Global Evolution (EDGE), it is expected that base stations will become commonplace that support the traffic equivalent of up to 20 TRX's per sector (in greater than 2×10 MHz bandwidths, and assuming full-rate AMR) with speeds of up to 473 kbps per EDGE TRX. Although these types of capacities and bandwidths are possible with the network architecture and building blocks of today's cell sites, they seem somewhat inefficient for meeting the high transmitter/receiver (TRX) demand and high backhaul requirements of these systems. Indeed, today's base transceiver station (BTS) deployments are entirely distributed where each one is dimensioned independently for the traffic load it carries in a given busy hour per day.
There are two fundamental areas that operators of mobile communication systems attempt to improve on over time, namely capital expenditures (CAPEX), and operating expenditures (OPEX). Under current deployment strategies, capital expenditures are expected to grow at a constant rate vs. an incremental minute-of-use (MoU) on the network. Unfortunately, the current base station architectures do not lend themselves towards improving OPEX as much as would be desired in a synthesized hopping network, since radios (with single channel amplifiers) must be combined using lossy combiners which results in a coverage loss unless additional coax cables are hung on the tower. In addition, every time more radios are added, the real estate footprint of the cell site itself grows as well, adding additional rent and lease expense (typical in those situations where outdoor shelters are utilized). Additionally, as Enhanced General Packet Switched Radio Service (EGPRS) packet data becomes fully deployed in the network, the number of effective T1 lines (i.e., the required transport bandwidth) between the base station controller (BSC) and the base transceiver station (BTS) is expected to increase significantly.
Due to these increasing OPEX costs, and no perceived improved CAPEX savings over time, it would be desirable to provide an alternative architecture to help reduce both CAPEX and OPEX for an evolved network. However, that can also have implications for timing advance timeout periods. Accordingly, it would be desirable to provide a centralized architecture that can also appropriately deal with timing advance and timeout issues.